1. Field of the Invention
The present invention relates to the image forming apparatus such as a copying machine or facsimile device.
2. Description of the Prior Art
A prior art copying machine usually has a control unit for controlling a light quantity in an original illumination light source in accordance with a sensitivity of a photosensitive drum and manually operated means for inputting a light quantity of accordance with the image density of an original.
It is very difficult to select a proper light quantity by such semi-automatic (substantially manual) means and a certain number of expensive copy forms have to be wasted by trial copies before a copy having a proper contrast is attained.
In order to resolve the above problem, copying machines having a function for detecting the original density of photo-sensing means or a surface potentiometer on the photosensitive drum to automatically control the light quantity have been proposed, but few of them discuss a way to satisfy a user's demand for controlling the light quantity in response to the detected original density (or copy density). Most of them which do discuss such control use a linear relation between the detected original density and the light quantity. In such an auto-exposure copying machine, it is usual to control the light quantity such that more light is applied to a higher density original and less light is applied to a lower density original, that is, a lighter copy is produced for a darker original and a darker copy is produced for a lighter original. However, when the linear relation of the density and the light quantity is used, a user's demand for the darker copy and the whiter copy is not satisfied.
In certain cases, only a portion of the original is detected to determing the light quantity. This results in inaccurate detection.
When a density of an entire area of the original is detected, a long time is required to determine a proper output and start the image formation.
A copying machine which controls an iris for the exposure to maintain a constant light quantity on a photo-sensitive plane without regard to a copy magnification factor has been proposed, but it is difficult to adjust the iris while taking the original density and the magnification factor into consideration.